For the purpose of position detection, it is known to provide a carrier that is movable in relation to a detector with markings that can be detected by the detector. On the basis of the markings detected by the detector, it is possible, with knowledge of the arrangement of the markings on the carrier, to make statements about the position and/or the movement of the carrier in relation to the detector. Position detection of this type is generally also referred to as incremental position detection.
The markings on the carrier may in this case be set up in such a way that they comprise items of information about the angular position at which they are situated, so that a statement about the position of the carrier can be made upon detection even of one marking. In this case, the information content of the individual markings may be so high that, with the detection just of a single marking, the position of the carrier can be detected precisely to one marking. Furthermore, the information content of the individual markings may also be lower, so that it is necessary first to detect a sequence of a few successive markings before a statement about the position of the carrier is possible.
Thus, by way of example EP 685 707 A1 discloses a method and an apparatus for detecting the angular position of a crankshaft, in the case of which a disk-shaped carrier is provided with an irregular pattern on the circumference, said pattern being detected by two magnetoresistive sensors. The pattern of markings on the carrier is furthermore stored as a reference sequence. For the purpose of position detection, a sequence of detected markings, which is shorter than the reference sequence, is compared with sections of the reference sequence until correspondence can be ascertained. This method disadvantageously results in an erroneous position detection as soon as the sensors no longer detect individual markings correctly.
U.S. Pat. No. 5,731,702 discloses an apparatus for detecting an annular position, in the case of which two parallel tracks of magnetic markings are provided on a wheel, said markings being sensed by two sensors for position detection. In this case, the output signals of the two sensors are monitored with regard to cable fracture or short circuit in order to identify errors during the detection of the markings. If one sensor has been identified as faulty, the output signal of the other sensor is evaluated with respect to time in order to obtain items of information about the position of the carrier. In this case, it is disadvantageous that an additional circuit is required for identifying a cable fracture or a short circuit and it is not possible to detect a small disturbance which, by way of example, on account of an altered distance between the marking and the detector, although it does not result in a recognizably impermissible signal in the detector, nevertheless results in an erroneous detection of the marking.
Furthermore, U.S. Pat. No. 4,782,329 discloses an apparatus for position detection, in the case of which the position of a carrier with markings is determined by means of a phase locked loop that is tracked to the detected markings, the markings representing a uniform sequence.
The present invention is based on the object of providing a method and an apparatus with which the certainty and reliability of the position detection can be increased.